مجله مهندسی مکانیک
سال چهلم شماره 2 (پیاپی 60، پاییز و زمستان 1389)

Amorphous Co68. 15Fe4. 35Si12. 5B15 ribbons were prepared by planar flow casting process and characterized by x-ray diffractometry (XRD)، differential thermal analysis (DTA) and magnetic measurements. Activation energy of amorphous to crystallization transition was estimated at various heating rates by two well-known Kissinger and Augis & Bennet (A&B) methods using DTA results، and was found to be around 673. 5 kJ/mol. The magnetic properties of amorphous ribbons heat treated under protective gas atmosphere at temperatures in the range of 300-560 °C for 15 to 60 minutes revealed that the annealing of as-spun ribbons up to 400 °C leads to an increase in the magnetic saturation induction، while severe deterioration of soft magnetic properties were observed with further increasing the temperature، due to formation of some crystalline phases، such as cobalt solid solution، Fe23B6 and Co13B7 phases.

Continuum damage mechanics is a powerful tool for predicting mechanical defects in the final products of metal forming processes، including tandem cold rolling. Generally speaking، there are various criteria to predict damage evolution in different processes. In the present study، the ductile damage model and the FLD criterion، as two widely used damage models، are employed and compared in order to predict strip tearing in tandem cold rolling process. Strip tearing is one of the manufacturing issues which can considerably raise the production costs. Therefore، it is necessary to develop an effective numerical model to predict strip tearing and analyze the effective factors which are related to this phenomenon. In this work، an industrial five-stand cold rolling mill is simulated using ABAQUS/Explicit finite element code. The ductile and FLD damage criteria are implemented and utilized to predict damage evolution through the strip. The damage parameters of material are determined from experiments. By comparison of numerical results with experimental observations، the more accurate and efficient damage model is selected and introduced. Finally the effect of the variations of angular velocity of the work rolls on damage evolution is investigated by applying the introduced damage model.

Present work investigates the effect of geometry، size and arrangement of ribs in an air channel on the heat transfer and friction at various regimes. The obstacles include the triangular، trapezoidal، rectangular، and circular riblets. The RNG k-ε turbulence model along with SIMPLE algorithm and second-order upwind method have been used for discretization of Navier-Stokes and energy equations. The numerical solutions have been compared with their experimental and numerical counterparts. The aim of this research is analyzing the performance of various riblets at different flow conditions and the behavior of numerical scheme.

Due to the bad cold-formability of aluminum alloys in deep drawing process، many forming methods have been explored to increase the drawing ratio of these alloys. Sheet Hydro-Forming is one of the processes that can be used to the forming of cups with high drawing ratio from bad cold-formability materials. In this paper، the forming of AA6063-T4 aluminum alloy hemispherical-cylindrical cups by hydrodynamic deep drawing assisted by radial pressure were investigated experimentally and analyzed using finite element simulation. By examining several pressure paths، the effect of pressure path on bursting and thickness distribution of the sheet was studied. The results indicated that by using this process and applying an appropriate pressure path، the sound cups، with high drawing ratio and uniform thickness distribution could be formed in one stage. Also، in this paper by using FE simulation and Taguchi technique، a systematically method was developed to analyze the effects of the forming parameters on the quality of part formability and determine the optimal combination of the forming parameters for sheet Hydro-Forming process. The results of analysis indicated that fluid pressure has the greatest influence on the part formability in sheet Hydro-Forming process. And also by using rough punch، lower gap between die rim block and blank holder and greater die entrance radius the formability of sheet will be improved.

Maraging Steels are a group of steels having high strength and toughness. Presence of inclusions in these steels are harmful for their strength and toughness. Also، there are some active elements within these steels. Therefore، application of advanced melting، refining and casting processes such as vacuum processes are used greatly in production of maraging steels. In this research two C300 maraging steel with 0. 65 and 1 wt% Ti were casted firstly in VIM furnace. Then samples subjected to VAR process in order to study the effects of this process on mechanical properties، inclusions content and gas atoms within the steel. The results show that during VAR process of the bars casted in VIM furnace، the total N and O gases reduced to 14%، ductility and resistane to impact increased up to 30% and 47%، respectively. Comparison of these two steels that produced by VIM+VAR showed that increasing of titanium from 0. 65 to 1 wt% causes marginal increasing in strength and hardness and marginal decreasing in ductility and resistance to impact of steel.

Thermo-mechanical analysis of a thick-walled cylinder with internal functionally graded (FGM) coating under thermal and mechanical loading along with the consideration of the effect of thermal residual stresses is carried out. Using multilayer assumption for the coating، and applying the continuity and equilibrium conditions at the interfaces، the axial forces and pressures at the interfaces and thermal residual stress in the cylinder and its coating are obtained. In order to perform a thermoelastic analysis using multilayer assumption، heat conduction and Navier equations are solved at each layer، and the transient thermal stress distributions in the cylinder and its coating are obtained.

Rarefied flow behavior which is often appears in relatively low pressures for macro sized devices, might also occur at atmospheric or even higher pressures for the applications with micron-sized characteristic lengths. Spherical particles such as fuel droplets injected in combustion systems can experience rarefied flow to some degrees due to their micron-size diameters. This rarefied flow behavior can have dramatic influences on particle dynamics and heat transfer rate. Therefore, in this paper flow over an isothermal sphere in the slip flow regime 10-3 Keywords: slip, sphere, heat transfer, drag coefficient, Nusselt number